All the symptoms - a rash, lethargy and deep purple bruising - pointed to meningococcal disease, but the expert medical team at Sydney Children's Hospital were quick to diagnose a rare infant blood disorder.

While the Watts were told to prepare for the worst, the care and treatment provided by the team over the following weeks ensured Cooper, now 4, enjoyed a full recovery.

The couple has since joined forces with other northern suburbs families, who all owe a debt of gratitude to the Sydney hospital, to put on an annual charity walk.

Organisers have set a $10,000 fund-raising target for the third annual Kids Walk For Life from Stanwell Park to Wollongong on Saturday.

"We nearly lost our son when he was seven months old and we owe so much to the team at Sydney Children's Hospital," Mr Watt said.

"In the two weeks we initially spent there, we made friends with so many people - with couples who had been there with their child for 18 months or more; with parents who didn't leave with their kids."

The Randwick hospital called in US experts to assist after Cooper was diagnosed with acute haemorrhagic oedema of infancy.

"We first noticed red spots behind Cooper's ears one morning, and then when we took off his jumpsuit we saw his feet were swollen and purple-black and immediately called the ambulance," Mr Watt said.

On Saturday, Cooper will join his parents and sister Madeline, 7, on the Kids Walk For Life.

Around 100 people are expected to take on the walk, which starts at 7am from the Stanwell Park kiosk and ends at Wollongong Harbour where buses will be waiting for the return trip.

Registrations will be taken prior to the walk while donations will be open for some weeks on www.gofundraise.com.au/page/walk3.